PROJECT SUMMARY: The major goal of the proposed research is to understand the molecular basis of lysosomal storage diseases, a collection of more than 40 inherited metabolic disorders that affect approximately 1 in 7,700 births. Lysosomal storage disorders are caused by defects in single genes, where the loss of a functional enzyme in the lysosome leads to accumulation of substrate and the development of disease symptoms. Lysosomal storage diseases are some of the best understood members of the larger protein-misfolding disease family, which includes Alzheimer's, Parkinson's, and Huntington's diseases. Because lysosomal storage diseases are generally caused by defects in single genes, the genetics of the diseases are simpler than in many human diseases. Lysosomal storage diseases are very active targets of clinical research, with enzyme replacement therapy, pharmacological chaperone therapy, substrate reduction therapy, bone marrow transplantation, gene therapy, and stem cell therapy approaches either approved or under clinical investigation. Despite hundreds of kilograms of recombinant enzymes produced industrially, the lack of basic knowledge about the structure, ligand binding, catalysis, and stability of the clinical targets has slowed clinical progress. For a lysosomal enzyme to function correctly, several critical steps must occur: the newly synthesized polypeptide must translocate into the Endoplasmic Reticulum (ER), where it must fold correctly; it must be post-translationally modified, allowing it to traffic through the Golgi apparatus to the lysosome; there, it must have the correct catalytic machinery and sufficient stability to perform its enzymatic task. A failure in any of these steps leads to loss of enzymatic function and subsequent disease progression. The understanding and treatment of lysosomal storage diseases has been limited by lack of understanding of the molecular defects that lead to disease symptoms. We will examine the basic biochemistry and biophysics at the root of lysosomal storage diseases, which will propel further translational progress on the diseases. To better understand the development of lysosomal storage diseases and other protein folding diseases, we propose to study the folding, stability, and function of lysosomal enzymes. We have developed methods for studying the biochemistry, biophysics, and cell biology of human lysosomal enzymes, allowing us to interrogate each stage in the maturation of the enzymes, from synthesis to trafficking to function in the lysosome. We are in the unique position to apply our expertise in the structural and cellular biology of human glycoproteins to the problem of lysosomal storage disorders. We have determined the three- dimensional structures of more human lysosomal enzymes than any other group, putting us in a unique position to study the basic biochemistry and biophysics of the family of proteins. By directly tackling difficult targets (human lysosomal enzymes are typically heavily glycosylated and otherwise post-translationally modified multimers) of high clinical significance, we advance the knowledge and treatment of disease.